Certain products, such as, for example, seals, can be produced from plastic workpieces by rotary machining on a lathe. The plastic workpieces here can be, for example, so-called semi-finished products, i.e., prefabricated objects. For example a semi-finished product can be a (thick-walled) plastic tube, which can be used for cutting out or cutting off of seal rings by rotary machining (i.e., processing during a rotating of the semi-finished product). During a rotary machining the semi-finished product, or generally speaking the workpiece, must be fixed. For this purpose clamping jaws are conventionally used in a jaw chuck. Here a plurality of clamping jaws are moved radially inward in order to radially contact an outer surface of the semi-finished product, e.g., a cylindrical outer surface, on a plurality of sides and to fix by clamping. Depending on the diameter of the semi-finished product, different jaw sizes must conventionally be used. When semi-finished products of various sizes must be processed, then after removal of the first semi-finished product, whose processing is to be ended, the associated clamping jaws are also removed from the lathe. Thereafter clamping jaws adapted according to the size of the second semi-finished product must be inserted into the lathe, and only thereafter can the second semi-finished product be fixed for the further processing by moving the further clamping jaws relative to one another. For example, for a diameter of approximately 30 mm to 400 mm of the semi-finished product, to date up to 18 clamping-jaw sets have been used. Here a clamping jaw set is comprised of three identical jaws that are each configured to contact an outer surface of the semi-finished product, e.g., a cylindrical outer surface, and to exert a pressing force in order to fix the semi-finished product.